This invention relates generally to a melter assembly for melting solid silicon, and more particularly to a melter assembly and method for charging a crystal forming apparatus with molten source material.
Single crystal material having a monocrystalline structure, which is the starting material for fabricating many electronic components such as semiconductor devices and solar cells, is commonly prepared using the Czochralski (“Cz”) method. Briefly described, the Czochralski method involves melting polycrystalline source material such as granular or chunk polycrystalline silicon (“polysilicon”) in a quartz crucible located in a specifically designed furnace to form a silicon melt. An inert gas such as argon is typically circulated through the furnace. A relatively small seed crystal is mounted above the crucible on a pulling shaft which can raise or lower the seed crystal. The crucible is rotated and the seed crystal is lowered into contact with the molten silicon in the crucible. When the seed begins to melt, it is slowly withdrawn from the molten silicon and starts to grow, drawing silicon from the melt having a monocrystalline structure.
Large grain polycrystalline semiconductor structures suitable for use as the starting material for the production of solar cells or other electrical components may be produced by various other processes known in the art. As with the Czochralski method, such alternative processes include various apparatus that utilize molten source material (e.g. silicon) to produce a solid crystalline body (e.g. ingot, ribbon, etc.) having desired electrical conduction properties. Such processes may include block casting which entails filling a cold crucible with molten silicon and allowing the molten silicon to solidify and form a polycrystalline body. Another process, commonly known as, the Edge-defined Film Growth (EFG) method involves growing hollow crystalline bodies in diverse shapes of controlled dimensions by using capillary die members which employ capillary action to assist the transfer of molten source material from a crucible to a seed crystal connected to a pulling apparatus. Also, various ribbon growth methods exist that involve the growth of a generally flat crystalline ribbon structure that is pulled from the melt of source material.
The various existing methods for forming semiconductor material for semiconductor devices and solar cells typically include the step of melting granular polysilicon directly in a crucible or adding a charge of molten silicon to a crucible. One drawback of melting granular polysilicon directly in a crucible is that the polysilicon is preferably highly pure, dehydrogenated silicon to reduce splatter from the release of hydrogen during melting. Splatter in the crucible causes silicon to deposit on the various components of the hot zone of the crystal forming apparatus and may result in impurities in the pulled crystal or damage to the graphite and silicon-carbide coated graphite components in the hot zone. Dehydrogenated chemical vapor deposition (CVD) granular polysilicon is expensive in comparison to more readily available CVD polysilicon that is not dehydrogenated, and its use adds to the production costs of silicon wafers or other electrical components produced by the various methods.
Additional operational and mechanical problems result from the melting of solid polysilicon in the main crucible of the crystal forming apparatus. For example, a large amount of power is required to melt the polysilicon due to its high thermal conductivity and high emissivity relative to liquid silicon. Also, melting solid polysilicon in the main crucible is time consuming typically requiring 15-18 hours to melt a single 250 kg (551 lbs) charge of polysilicon. Further, the thermal stresses (both chemical and mechanical) induced in the crucible by exposure to the high melting temperatures required to melt the solid polysilicon cause particles of the crucible walls to be loosened and suspended in the melt resulting in lower crystal quality and premature failure of the crucible. Also, the crucible is subjected to mechanical stresses from the loading of solid polysilicon particles that frequently scratch or gouge the crucible wall resulting in damage to the crucible and removal of particles from the crucible walls that may contaminate the silicon melt and the bodies formed therefrom.
Various prior art methods have attempted to eliminate the requirement of melting polysilicon in a crucible. These prior art methods include providing an auxiliary crucible for melting the polysilicon located above the main crucible so that gravity feed with or without use of differential pressure allows molten silicon to flow into the main crucible during crystal growth. These existing prior art methods do not efficiently and quickly melt the solid polysilicon and do not transfer the melted silicon to the main crucible in a manner that reduces melt splatter on the hot zone parts of the crystal forming apparatus. Also, the existing methods do not provide a quick and economical way of heating the solid polysilicon to reduce melting time and increase the throughput of the crystal forming apparatus. Therefore, a need exists for a method of supplying molten silicon to a crystal forming apparatus that quickly and efficiently melts the solid polysilicon and transfers the molten silicon to the main crucible of the apparatus in a way that reduces splatter and maintains the quality of the resulting silicon crystal.